Electrochemical nitridation of a stainless steel for PEMFC bipolar plates

AISI446 steel has been electrochemically nitrided in 0.1 M HNO₃ + 0.5 M KNO₃ solution at room temperature. XPS analysis revealed surface NH₃ and a deeper nitride layer. The surface layer of the stainless steel modified by electrochemical nitridation was thus composed of a nitrogen-incorporated oxide film. The nitrided steel showed very low interfacial contact resistance (ca. 18 mΩ cm² at 140 N/cm²) and excellent corrosion resistance in simulated PEMFC environments. Electrochemical nitridation provides an economic way to modify the stainless steel’s surface, and is very promising for application to fuel cell bipolar plates.     

Chromium nitride/Cr coated 316L stainless steel as bipolar plate for proton exchange membrane fuel cell

Chromium nitride/Cr coating has been deposited on surface of 316L stainless steel to improve conductivity and corrosion resistance by physical vapor deposition (PVD) technology. Electrochemical behaviors of the chromium nitride/Cr coated 316L stainless steel are investigated in 0.05 M H₂SO₄ + 2 ppm F⁻ simulating proton exchange membrane fuel cell (PEMFC) environments, and interfacial contact resistance (ICR) are measured before and after potentiostatic polarization at anodic and cathodic operation potentials for PEMFC. The chromium nitride/Cr coated 316L stainless steel exhibits improved corrosion resistance and better stability of passive film either in the simulated anodic or cathodic environment. In comparison to 316L stainless steel with air-formed oxide film, the ICR between the chromium nitride/Cr coated 316L stainless steel and carbon paper is about 30 mΩ cm² that is about one-third of bare 316L stainless steel at the compaction force of 150 N cm⁻². Even stable passive films are formed in the simulated PEMFC environments after potentiostatic polarization, the ICR of the chromium nitride/Cr coated 316L stainless steel increases slightly in the range of measured compaction force. The excellent performance of the chromium nitride/Cr coated 316L stainless steel is attributed to inherent characters. The chromium nitride/Cr coated 316L stainless steel is a promising material using as bipolar plate for PEMFC.     

Effect of plasma nitriding on behavior of austenitic stainless steel 304L bipolar plate in proton exchange membrane fuel cell

A dense and supersaturated nitrogen layer with higher conductivity is obtained on the surface of austenitic stainless steel 304L by the low temperature plasma nitriding. The effect of plasma nitriding on the corrosion behavior and interfacial contact resistance (ICR) for the austenitic stainless steel 304L was investigated in 0.05 M H₂SO₄ + 2 ppm F⁻ simulating proton exchange membrane fuel cell (PEMFC) environment using electrochemical and electric resistance measurements. The experiment results show that the stable passive film is formed after the potentiostatic polarization at the specified anodic or cathodic potentials under PEMFC operation condition, and the plasma nitriding improves slightly the corrosion resistance and decreases markedly the ICR of 304L. The ICR of the plasma nitrided 304L increases after the potentiostatic polarizations for 4 h, and lower than 100 mΩ cm² at the compaction force of 150 N cm⁻².     

Corrosion protection of 304 stainless steel bipolar plates using TiC films produced by high-energy micro-arc alloying process

Metallic bipolar plates look promising for the replacement of graphite due to higher mechanical strength, better durability to shocks and vibration, no gas permeability, acceptable material cost and superior applicability to mass production. However, the corrosion and passivation of metals in environments of proton exchange membrane fuel cell (PEMFC) cause considerable power degradation. Great attempts were conducted to improve the corrosion resistance of metals while keeping low contact resistance. In this paper, a simple, novel and cost-effective high-energy micro-arc alloying process was employed to prepare compact titanium carbide as coatings for the type 304 stainless steel bipolar plates with a metallurgical bonding between the coating and substrate. It was found that TiC coating increased the corrosion potential of the bare steel in 1 M H₂SO₄ solution at room temperature by more than 200 mV, and decreased significantly its corrosion current density from 8.3 μA cm⁻² for the bare steel to 0.034 μA cm⁻² for the TiC-coated steel. No obvious degradation was observed for the TiC coatings after 30-day exposure in solution.     

Improved anticorrosion properties and electrical conductivity of 316L stainless steel as bipolar plate for proton exchange membrane fuel cell by lower temperature chromizing treatment

The lower temperature chromizing treatment is developed to modify 316L stainless steel (SS 316L) for the application of bipolar plate in proton exchange membrane fuel cell (PEMFC). The treatment is performed to produce a coating, containing mainly Cr-carbide and Cr-nitride, on the substrate to improve the anticorrosion properties and electrical conductivity between the bipolar plate and carbon paper. Shot peening is used as the pretreatment to produce an activated surface on stainless steel to reduce chromizing temperature. Anticorrosion properties and interfacial contact resistance (ICR) are investigated in this study. Results show that the chromized SS 316L exhibits better corrosion resistance and lower ICR value than those of bare SS 316L. The chromized SS 316L shows the passive current density about 3E−7 A cm⁻² that is about four orders of magnitude lower than that of bare SS 316L. ICR value of the chromized SS 316L is 13 mΩ cm² that is about one-third of bare SS 316L at 200 N cm⁻² compaction forces. Therefore, this study clearly states the performance advantages of using chromized SS 316L by lower temperature chromizing treatment as bipolar plate for PEMFC.     

Conductive amorphous carbon-coated 316L stainless steel as bipolar plates in polymer electrolyte membrane fuel cells

Amorphous carbon (a-C) film about 3 μm in thickness is coated on 316L stainless steel by close field unbalanced magnetron sputter ion plating (CFUBMSIP). The AFM and Raman results reveal that the a-C coating is dense and compact with a small size of graphitic crystallite and large number of disordered band. Interfacial contact resistance (ICR) results show that the surface conductivity of the bare SS316L is significantly increased by the a-C coating, with values of 8.3–5.2 mΩ cm² under 120–210 N/cm². The corrosion potential (E_corr) shifts from about −0.3 V vs SCE to about 0.2 V vs SCE in both the simulated anode and cathode environments. The passivation current density is reduced from 11.26 to 3.56 μA/cm² with the aid of the a-C coating in the simulated cathode environment. The a-C coated SS316L is cathodically protected in the simulated anode environment thereby exhibiting a stable and lower current density compared to the uncoated one in the simulated anode environment as demonstrated by the potentiostatic results.     

The bipolar plate of AISI 1045 steel with chromized coatings prepared by low-temperature pack cementation for proton exchange membrane fuel cell

The low-temperature pack chromization, a reforming pack cementation process, is employed to modify AISI 1045 steel for the application of bipolar plates in PEMFC. The process is conducted to yield a coating, containing major Cr-carbides and minor Cr-nitrides, on the substrate in view of enhancing the steel's corrosion resistance and lowering interfacial contact resistance between the bipolar plate and gas diffusion layer. Electrical discharge machining and rolling approach are used as the pretreatment to produce an activated surface on the steel before pack chromization process to reduce operating temperatures and increase deposition rates. The rolled-chromized steel shows the lowest corrosion current density, 3 × 10⁻⁸ A cm⁻², and the smallest interfacial contact resistance, 5.9 mΩ cm², at 140 N cm⁻² among all tested steels. This study clearly states the performance of 1045 carbon steel modified by activated and low-temperature pack chromization processes, which possess the potential to be bipolar plates in the application of PEMFC.     

Effects of thermal oxi-nitridation on the corrosion resistance and electrical conductivity of 446M stainless steel for PEMFC bipolar plates

Stainless steel has attracted interest as a bipolar plate material for polymer electrolyte membrane fuel cells due to its excellent mechanical properties, good corrosion resistance, and low cost. However, the application of thermal nitridation for the improvement of electrical conductivity deteriorates the corrosion resistance under PEMFC operating conditions due to the discontinuous formation of external Cr-nitride. In this study, nitridation with pre-oxidation of 446M stainless steel was performed in order to improve both the corrosion resistance and the electrical conductivity. 446M stainless steels with oxide and nitride on the surface were evaluated to assess their feasibility as a bipolar plate material for PEMFCs. The results were compared with those obtained using as-received and only nitrided 446M stainless steels. The oxide formed by the pre-oxidation protects the surface of 446M stainless steel from corrosion in corrosive environments, especially under cathode conditions, and the Cr-nitride formed by the subsequent nitridation serves as an electro-conductive channel. As a result, the pre-oxidized, nitrided 446M stainless steel exhibits improved corrosion properties and electrical conductivity under PEMFC operating environments.     

Inductively coupled plasma nitriding of chromium electroplated AISI 316L stainless steel for PEMFC bipolar plate

Chromium electroplated AISI 316L stainless steel was nitrided using inductively coupled plasma (ICP) for application in the bipolar plate of a polymer electrolyte membrane fuel cell (PEMFC). A continuous and thin chromium nitride layer was formed at the surface of the samples after ICP nitriding for 2 h at 400 °C. The interfacial contact resistance (ICR) and corrosion resistance in simulated PEMFC operating conditions were higher than the required values, while they varied with the applied dc bias voltage during the nitriding process. The ICR value decreased with an increase in bias voltage. Potentiodynamic polarization measurements showed that all of the nitrided samples had excellent corrosion resistance with a current density of ∼10⁻⁷ A cm⁻² at the cathode. It was also found that the oxygen content at the surface was not increased after the corrosion test. X-ray diffractometry (XRD), field emission scanning electron microscopy (FE-SEM), and Auger electron spectroscopy (AES) were used to analyze the effect of plasma nitriding.     

Improvement of corrosion resistance and electrical conductivity of 304 stainless steel using close field unbalanced magnetron sputtered carbon film

Carbon film has been deposited on 304 stainless steel (SS304) using close field unbalanced magnetron sputter ion plating (CFUBMSIP) to improve the corrosion resistance and electrical conductivity of SS304 acting as bipolar plates for proton exchange membrane fuel cells (PEMFCs). The corrosion resistance, interfacial contact resistance (ICR), surface morphology and contact angle with water of the bare and carbon-coated SS304 are investigated. The carbon-coated SS304 shows good corrosion resistance in the simulated cathode and anode PEMFC environment. The ICR between the carbon-coated SS304 and the carbon paper is 8.28-2.59 mΩ cm² under compaction forces between 75 and 360 N cm⁻². The contact angle of the carbon-coated SS304 with water is 88.6°, which is beneficial to water management in the fuel cell stack. These results indicate that the carbon-coated SS304 exhibits high corrosion resistance, low ICR and hydrophobicity and is a promising candidate for bipolar plates.     

High-density plasma nitriding of AISI 316L for bipolar plate in proton exchange membrane fuel cell

Austenitic stainless steel (AISI 316L) is nitrided by inductively coupled plasma using a gas mixture of N₂ and H₂ at temperatures between 530 K and 650 K, and the corrosion resistance as well as the interfacial contact resistance (ICR) are measured in a simulated proton exchange membrane fuel cell (PEMFC) environment.After plasma nitriding, a nitrogen-expanded austenite layer, the so-called S-phase is formed in all nitrided samples. The ICR value of the nitrided samples decreases to approximately 10 mΩcm² after plasma nitriding. The sample nitrided at 590 K shows the best corrosion property, while the corrosion resistance of the sample nitrided at higher temperatures decreases because of the formation of Cr-depleted regions in the nitrided sample. By using high-density plasma, the process temperature can be reduced to such a low temperature that Cr depletion is not significant, but a dense S-phase is formed.     

Coated 316L stainless steel with CrxN film as bipolar plate for PEMFC prepared by pulsed bias arc ion plating

Three different kinds of Cr_xN films on 316L stainless steels were prepared by pulsed bias arc ion plating as bipolar plates for proton exchange membrane fuel cell (PEMFC). The interfacial contact resistance, corrosion resistance and surface energy of the bipolar plate samples were investigated. Among the three samples, the 316L stainless steel coated with Cr_0.49N_0.51 → Cr_0.43N_0.57 gradient film (sample 2) exhibited the best-integrated performance. The contact resistance between sample 2 and Toray carbon paper was 6.9–10.0 mΩ cm² under 0.8–1.2 MPa. The bipolar plate sample also showed improved corrosion resistance in simulated PEMFC environments. Either in the reduction environment or in the oxidation environment 25 °C and 70 °C, the corrosion current densities of sample 2 were about one to two orders of magnitude lower than those of the base metal. In addition, the open circuit corrosion potential of sample 2 was also the highest in 0.5 M H₂SO₄ + 5 ppm F⁻ solution at 25 °C. The treated bipolar plate had high surface energy; and the contact angle of sample 2 with water was about 90°, which is beneficial for water management in fuel cell.     

Surface modification and performance of inexpensive Fe-based bipolar plates for proton exchange membrane fuel cells

A reforming pack chromization with rolling pretreatment process is utilized to develop inexpensive and high-performance Fe-based metal bipolar plates (SS 420, SS 430, and SS 316 stainless steels) for PEMFC systems. Rolling process is previously performed to reduce the chromizing temperature and generate a coating possessing excellent conductivity and corrosion resistance on the steels during chromization. The power efficiencies of rolled-chromized and simple chromized bipolar plates are compared with graphite bipolar plates employed in PEMFCs. The results show that the rolled-chromized bipolar plates have a corrosion current (I_corr) of 7.87 × 10⁻⁸ A cm⁻² and an interfacial contact resistance of 9.7 mΩ cm². Moreover, the power density of the single cell assembled with rolled-chromized bipolar plates is 0.46 W cm⁻², which is very close to that of graphite (0.50 W cm⁻²), in the tested conditions of this study.     

Nanocomposite-carbon coated at low-temperature: A new coating material for metallic bipolar plates of polymer electrolyte membrane fuel cells

A nanocomposite-carbon layer is coated onto the surface of 316L stainless steel (SS316) using a beam of accelerated C₆₀ ions at low temperature. The coating is composed of textured graphite nanocrystals ranging in size from 1 to 2 nm, with the graphene plane normal to the coating plane; the nanocrystals are separated by amorphous carbon. This orientation of the graphene layer provides low film resistivity in the direction of the substrate normal. Corrosion resistance tests performed in aggressive anodic and cathodic environments of a polymer electrolyte membrane fuel cell (PEMFC) show that the nanocomposite-carbon coated SS316L exhibits better anticorrosion properties than does bare SS316L. The interfacial contact resistance (ICR) of the nanocomposite-carbon coated SS316L is 12 mΩ cm², which is similar to that of graphite at a compaction force of 150 N cm⁻² and lower than a target of ∼20 mΩ cm². A low value of ICR is maintained even after corrosion tests in aggressive anodic and cathodic environments. The fabricated nanocomposite-carbon coated SS316L exhibits excellent corrosion resistance and low interfacial contact resistance under simulated PEMFC bipolar plate conditions.     

Effect of conducting composite polypyrrole/polyaniline coatings on the corrosion resistance of type 304 stainless steel for bipolar plates of proton-exchange membrane fuel cells

A bilayer conducting polymer coating composed of an inner layer of polypyrrole (Ppy) with large dodecylsulfate ionic groups obtained by galvanostatic deposition, and an external polyaniline (Pani) layer with small SO₄²⁻ groups obtained by cyclic voltammetric deposition was prepared to protect type 304 stainless steel used for bipolar plates of a proton-exchange membrane fuel cell. The corrosion performance of the bare and coated steel in 0.3 M HCl was examined by electrochemical impedance spectroscopy, polarization and open-circuit potential measurements. The experimental results indicated that both the composite Ppy/Pani coatings and the single Ppy coatings increased the corrosion potential of the bare steel by more than 400 mV (saturated calomel electrode), and increased the pitting corrosion potential by more than 500 mV (saturated calomel electrode). The bilayer coatings could reduce the corrosion of the alloy much more effectively than the single Ppy coatings, serving as a physical barrier and providing passivity protection, with acceptable contact resistance.     

An investigation into TiN-coated 316L stainless steel as a bipolar plate material for PEM fuel cells

In order to reduce the cost, weight and volume of the bipolar plates, considerable attention is being paid to developing metallic bipolar plates to replace the non-porous graphite bipolar plates that are in current use. However, metals are prone to corrosion in the proton exchange membrane (PEM) fuel cell environments, which decreases the ionic conductivity of the membrane and lowers the overall performance of the fuel cells. In this study, TiN was coated on SS316L using a physical vapor deposition (PVD) technology (plasma enhanced reactive evaporation) to increase the corrosion resistance of the base SS316L. X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical methods were used to characterize the TiN-coated SS316L. XRD showed that the TiN coating had a face-centered-cubic (fcc) structure. Potentiodynamic tests and electrochemical impedance tests showed that the corrosion resistance of SS316L was significantly increased in 0.5 M H₂SO₄ at 70 °C by coating with TiN. In order to investigate the suitability of these coated materials as cathodes and anodes in a PEMFC, potentiostatic tests were conducted under both simulated cathode and anode conditions. The simulated anode environment was −0.1 V versus SCE purged with H₂ and the simulated cathode environment was 0.6 V versus SCE purged with O₂. In the simulated anode conditions, the corrosion current of TiN-coated SS316L is −4 × 10⁻⁵ A cm⁻², which is lower than that of the uncoated SS316L (about −1 × 10⁻⁶ A cm⁻²). In the simulated cathode conditions, the corrosion current of TiN-coated SS316L is increased to 2.5 × 10⁻⁵ A cm⁻², which is higher than that of the uncoated SS316L (about 5 × 10⁻⁶ A cm⁻²). This is because pitting corrosion had taken place on the TiN-coated specimen.     

High nitrogen stainless steel as bipolar plates for proton exchange membrane fuel cells

To investigate the applicability of high nitrogen (HN) austenitic stainless steel as bipolar plates for proton exchange membrane fuel cells (PEFCs), the polarization tests were carried out in synthetic solutions (0.05 M SO₄²⁻ (pHs 2.3, 4.3 and 5.5) +2 ppm F⁻) at 353 K. Interfacial contact resistance between the stainless steel and gas diffusion layer was measured before and after polarization. A single cell employing the HN stainless steel as bipolar plates was operated for 1000 h at 0.5 A cm⁻² (12.5 A). The single cell exhibited voltage drop of 17 mV during the operation. Corrosion products were scarcely detected for the HN stainless steel bipolar plate, as confirmed by scanning electron microscopy. After the polarization tests and single cell operation, XPS analyses were carried out to examine the resulting surface states. In the synthetic solutions to pH 4.3, the passive films mainly consisted of oxides enriched with Cr. When the solution pH was 5.5, on the other hand, the films were mainly composed of Fe-oxides. After the single cell operation for 1000 h, it was found that the passive films of the rib surface for the gas inlet part was mainly composed by Fe-oxides. On the other hand, the passive films for the ribs from center to gas outlet part were mainly made up of Cr-oxides. By combining the simulated and real operation environments, it is believed that the corrosion resistive Cr-oxides passive layer of the HN stainless steel obtained by the presence of nitrogen incorporated into the stainless steel could contribute to the maintenance of the higher cell voltage during the extensive cell operation.     

Pre-oxidized and nitrided stainless steel alloy foil for proton exchange membrane fuel cell bipolar plates: Part 1. Corrosion, interfacial contact resistance, and surface structure

Thermal (gas) nitridation of stainless steel alloys can yield low interfacial contact resistance (ICR), electrically conductive and corrosion-resistant nitride containing surface layers (Cr₂N, CrN, TiN, V₂N, VN, etc.) of interest for fuel cells, batteries, and sensors. This paper presents results of scale-up studies to determine the feasibility of extending the nitridation approach to thin 0.1 mm stainless steel alloy foils for proton exchange membrane fuel cell (PEMFC) bipolar plates. Developmental Fe-20Cr-4V alloy and type 2205 stainless steel foils were treated by pre-oxidation and nitridation to form low-ICR, corrosion-resistant surfaces. As-treated Fe-20Cr-4V foil exhibited target (low) ICR values, whereas 2205 foil suffered from run-to-run variation in ICR values, ranging up to 2× the target value. Pre-oxidized and nitrided surface structure examination revealed surface-through-layer-thickness V-nitride particles for the treated Fe-20Cr-4V, but near continuous chromia for treated 2205 stainless steel, which was linked to the variation in ICR values. Promising corrosion resistance was observed under simulated aggressive PEMFC anode- and cathode-side bipolar plate conditions for both materials, although ICR values were observed to increase. The implications of these findings for stamped bipolar plate foils are discussed.     

Preparation and performance of nanostructured porous thin cathode for low-temperature solid oxide fuel cells by spin-coating method

To reduce the cathode–electrolyte interfacial polarization resistance of low-temperature solid oxide fuel cells (SOFCs), a nanostructured porous thin cathode consisting of Sm_0.5Sr_0.5CoO₃ (SSC) and Ce_0.8Sm_0.2O_1.9 (SDC) was fabricated on an anode-supported electrolyte film using spin-coating technique. A suspension with nanosized cathode materials, volatilizable solvents and a soluble pore former was developed. The results indicated that the cell with the nanostructured porous thin cathode sintered at 950 °C showed relatively high maximum power density of 212 mW cm⁻² at 500 °C and 114 mW cm⁻² at 450 °C, and low interfacial polarization resistance of 0.79 Ω cm² at 500 °C and 2.81 Ω cm² at 450 °C. Hence, the nanostructured porous thin cathode is expected to be a promising cathode for low-temperature SOFCs.     

Titanium oxynitride films for a bipolar plate of polymer electrolyte membrane fuel cell prepared by inductively coupled plasma assisted reactive sputtering

Titanium oxynitride (TiN_xO_y) films are investigated for application as a bipolar plate coating material in a polymer electrolyte membrane fuel cell (PEMFC). TiN_xO_y films with various amounts of oxygen are deposited on stainless-steel substrates by inductively coupled plasma (ICP) assisted reactive sputtering by changing the oxygen gas flow rate. The interfacial contact resistance (ICR) and the corrosion resistance of the TiN_xO_y films are measured under PEMFC simulated conditions. When the amount of oxygen in the TiN_xO_y film is approximately <12 at.% (O₂ flow rate ≤0.2 sccm), the corrosion resistance is enhanced considerably, whereas the interfacial contact resistance does not change. The corrosion current density decreases from 8 × 10⁻⁶ A cm⁻² for the TiN-coated sample to 2.7 × 10⁻⁶ A cm⁻² at 0.6 V vs. SCE as a result of oxygen incorporation in the TiN film. The ICR value remains at 2.5 mΩ cm² at 150 N cm⁻². When a small amount of oxygen is added to the TiN film, it is postulated that the oxygen atoms locate at the column and grain boundaries, and thus prevent corrosive media from penetrating into the substrate while not deteriorating the electrical property of the film.